There is little doubt that the majority of wines significantly benefit from the malolactic fermentation, through a reduction in acidity and the development of subtle flavors that contribute to the complexity of sensory quality. Moreover, the vast majority of wine makers will not bottle their wines, particularly their red wines, until this reaction has been completed because the risk of this fermentation occurring naturally in the bottle is too great and, in that circumstance, the wine would be ruined. For these reasons, the vast majority of wine makers in Australia and other countries prefer to have all of their red wines and, now many of their white wines undergo malolactic fermentation prior to bottling.
Many wine makers still rely on the traditional method of naturally occurring malolactic fermentation, as a consequence of the growth of lactic acid bacteria (principally Leuconostoc oenos) indigenous in the wine. The unpredictability of this natural development has been a major worry to the wine maker. In some circumstances, the reaction would occur within 2-4 weeks after alcoholic fermentation whereas in others it might not occur for several weeks, months or not at all. When it does occur 2-4 weeks are generally required for bacteria to grow and complete the fermentation. Failure of the reaction to occur, delays further processing of the wine and is a substantial inconvenience to wine making. Because of this, there has been a strong move by wine makers in recent years to purposely encourage or induce the development of the malolactic fermentation by inoculating cultures of Leuconostoc oenos into the wine. Such cultures are either purchased commercially or prepared in large volume by the wine maker. In large wineries, it becomes expensive to purchase sufficient volumes of culture so that further cultivation of suitable organisms is often necessary to generate the quantity of biomass needed for inoculation into the wine.
Although inoculation substantially increases the probability the malolactic fermentation will occur, there is still an element of unpredictability and there is no 100% guarantee that the fermentation will occur. This is because the inoculated species must grow in the wine to conduct the fermentation and in some circumstances, the conditions of the wine are not supportive of this growth.
Wine industry trends towards induction of malolactic fermentation using inoculated fermentations have been restricted because the following problems still remain with this technique:
1) purchasing a large scale production of Leuconostoc oenos or other suitable biomass is expensive and demanding of winery resources; PA1 2) fermentation relies upon growth of the inoculated cells, at best this is slow, requiring 2-4 weeks for a completed reaction and in some circumstances it might not occur at all giving no malolactic fermentation. PA1 a) passing wine containing malic acid through a bacteria retaining filter into a reaction vessel containing a culture of a bacterium or yeast capable of converting malic acid to lactic acid having a cell concentration of at least 10.sup.8 cfu/ml (colony forming units per ml). PA1 b) allowing the wine a residence time in the reaction vessel sufficient to enable the bacteria or yeast to convert at least a proportion of the malic acid in the wine to lactic acid; and PA1 c) withdrawing wine from the reaction vessel through a bacteria retaining filter. PA1 a) a reactor vessel; PA1 b) inlet means for introducing wine into the reaction vessel; PA1 c) a bacteria retaining filter to filter wine entering the reaction vessel through the inlet means; PA1 d) an outlet means for withdrawing wine from the reaction vessel; PA1 e) a bacteria retaining filter unit to filter wine being withdrawn from the reaction vessel through the outlet means; and PA1 f) the reaction vessel containing a cell suspension containing a concentration of a bacterium or yeast capable of converting malic to lactic acid containing at least 10.sup.8 cfu/ml.
It is known that high concentrations of from 10.sup.6 to 10.sup.8 cells per ml of cells of Leuconostoc oenos or other suitable bacteria will rapidly convert malic acid to lactic acid in wine without the need for cell growth. The reaction occurs in a matter of minutes.
Under these conditions, the cells of Leuconostoc oenos act as a biocatalyst. Attempts have been made towards the development of technology for the rapid, continuous malolactic fermentation of wine. In these previous cases, the high cell concentrations of Leuconostoc oenos were attached to a solid support and the wine passed continuously over this support. While the desired objective (the rapid degradation of malic acid to lactic acid) was achieved, these technologies were never commercially developed. This is due to problems associated with microbial contamination of the reactors, transfer of flavor taints to the wine, loss of activity of cells on prolonged operation and leakage of cells from the solid support. Furthermore, these technologies were not properly evaluated for their effect on sensory properties of the wine.
Several researchers have recognized the commercial prospects of conducting continuous deacidification of wine with high concentrations of non-growing, cells of Leuc. oenos that could be immobilized on a solid support and packed into a column (Davis et al. 1985b Am. J. Enol. Vitic 36: 290-301). Cuenat and Villettaz ((1984) Revise Suisse Vitic. Arboric Hortic 16: 145-151) immobilized a strain of Leuc. oenos on alginate and were able to completely remove malic acid in several wines of low and high SO.sub.2 concentration in a continuous system. For example, two 2.5 L columns of immobilized cells connected in series were used to deacidify a Marsanne wine containing 11.3% ethanol and 139 mg/L total SO.sub.2 at pH 3.3. Twenty five liters of wine were passed through the columns in 5.5 hr, after which the initial concentration of malic acid (6.1 g/L) was reduced to 0.07 g/L. Although immobilized cells may offer practical advantages in the deacidification of wines, such technology has not been commercially developed. The reasons for this are not clear but could relate to problems associated with microbial contamination of the reactors, transfer of flavor taints to the wine, loss of activity on prolonged operation and leakage of cells from the solid support. Furthermore, these technologies were not properly evaluated for their effect on the sensory properties of the wine (Davis et al. 1985b).
The present inventors have now surprisingly found that very high concentrations of free or immobilized lactic acid bacteria may be used to continuously achieve rapid conversion of malic acid to lactic acid without substantial adverse effect to the wine.
Further, surprisingly it has been found that the bacteria can remain biocatalitically active for an indefinite time, thus allowing the process of the invention to operate continuously, or with interruptions.